1. Field of the Invention
The present invention relates to a method and apparatus for data transmission by a mobile station in a Time Division Multiple Access (TDMA) system. More specifically, the invention concerns multi-slot transmission of data services in such a system.
2. Description of the Prior Art
TDMA is a fundamental digital telecommunication technology and forms the basis of cellular standards such as Global System for Mobile Communications (GSM). A TDMA system includes the basic subsystems of cellular radio systems including a switching network, base stations and mobile stations (MS). In a TDMA system the radio spectrum is divided into radio carrier frequencies typically spaced 30 kHz to 200 kHz apart. This spacing between the carrier frequencies is the nominal or effective bandwidth of the total multi-channel multiplexed signal. Digital techniques are employed at the base station and in the cellular radio to subdivide the time on each radio channel into time slots, i.e. the TDMA radio carrier waveform is divided into several different types of control and voice/data channels by the use of different time slots or shared portions of time slots. Time slots are the smallest individual time periods available to each mobile station. Each time slot can be assigned to a different mobile telephone, and the time slots can be dedicated or dynamically assigned. Voice or data information as well as access information are converted to digital information that is sent and received in bursts during the time slots. The burst of digital information can be encoded, transmitted, and decoded in a fraction of the time required to produce the sound. Therefore, only a fraction of the airtime is used by one channel, and other subscribers can use the remaining time slots on the radio channel. The result is that TDMA systems allow several mobile stations (MS) to operate simultaneously on a single radio carrier frequency because the mobile stations share the radio frequency by dividing their signals into slots.
In the GSM system, eight mobile telephones are able to share a single 200 kHz bandwidth radio carrier waveform for voice or data communications. OSM uses a type of radio channel called a traffic channel (TCH) which carries voice, data, and control information. The TCH is organized into frames and time slot bursts, The entire repeating pattern of time slots is called a frame and comprises eight time slots assigned to eight different users. From each frame, each user is assigned to a particular time slot burst for reception, and a particular corresponding burst for transmission. Hence, GSM multiplexes (time shares) several users onto a single radio carrier frequency through use of distinct time slots from each frame to individual users. Thus the GSM radio channel structure allows multiple mobile stations to communicate on the same frequency by using different time slots on the radio channel.
As is well understood, such intensified use of radio spectrum greatly multiplies subscriber capacity. A further advantage of digital cellular technology is its ability to support advanced high speed data services such as short message service (SMS), broadcast paging, imaging services and fax transmission.
Data services ordinarily are communicated in the same way as voice signals, that is they are assigned a particular individual time slot in a frame and share the single bandwidth radio carrier with other users, However, whilst the transfer of voice signals must be on a real time basis (instantaneous), certain data services can be transferred on a non-real time basis (stored or delayed). For example because paging messages can be delayed for several minutes without significant disadvantages to the receiver of the message, it allow s short paging messages to be placed in a queuing system. Voice services on the other hand cannot queue an incoming call for more than a few seconds otherwise it is likely that the caller would terminate the call.
Non-real time operability of data services allows transmission of the information to be treated with increased flexibility. This has led recently to the development of multi-slot data transfer capability. Multi-slot transmission is facilitated by the network, typically the base station, which controls channel allocation to users. If more than one slot in a frame is available (for instance during a period of relative inactivity and the slots have not be allocated to other users) then the network may designate the available slots to the transfer of data services. Accordingly, the network will command the mobile station to transmit the data communication on more than one consecutive slot. The data can then be decoded from the multi-slot transmission in the network.